Linear low density polyethylene (LLDPE) resins represent a very large and important segment of the polyethylene (PE) blown and cast film markets worldwide. These resins are synthesized by copolymerizing ethylene with an alpha-olefin comonomer such as 1-butene, 1-hexene or 1-octene. This results in an ethylene/alpha-olefin copolymer with many short chain branches (SCB) along the polymer backbone. The incorporation of 1-butene, 1-hexene or 1-octene comonomers results in ethyl (2 carbons), butyl (4 carbons) or hexyl (6 carbons) branches, respectively, along the polymer backbone.
Chain length of the short chain branches has significant effects on the end use properties and processability. The effects of branching on the properties of polyethylene depend on the length and the amount of the branches. Short chain branches (SCB), of less than approximately 40 carbon atoms, interfere with the formation of the crystal structure. Short branches mainly influence the mechanical and thermal properties. It was observed that differences in comonomer type have profound effects on the blown film properties. In general, film performance properties such as impact (toughness) and tear strengths increase with an increase in the comonomer length. Blown film performance is also greatly influenced by the comonomer composition distribution (CCD) (also often referred to as the short chain branch distribution (SCBD)) across the molecular weight distribution (MWD).
LLDPE can benefit from the addition of longer-chain comonomers. Many have been interested in modifying the architecture of such polyolefins in the hopes of obtaining new and better combinations of properties. One method of controlling polyolefin architecture is to select comonomers that will impart specific characteristics or tailoring the comonomers used. For example, several have tried to produce large “monomers” called “macromonomers” or “macromers” having amounts of vinyl, vinylidene or vinylene termination that can be polymerized with ethylene to impart longer chain branching, structural properties, etc. to a polyolefin. Typically, vinyl macromonomers are found more useful or easier to use than vinylene or vinylidene macromonomers. Examples of methods to produce various vinyl terminated macromonomers are disclosed in U.S. Pat. No. 6,117,962; U.S. Pat. No. 6,555,635; Small, Brookhart, Bennett, J Am Chem Soc 120, 1998, p. 4049; and Britovsek, et al. Chem. Comm. 1998, p. 849; Su, et al. Organomet., 25, 2006, p. 666. See also B. L. Small and M. Brookhart, “Polymerization of Propylene by a New Generation of Iron Catalysts: Mechanisms of Chain Initiation, Propagation, and Termination” Macromol., 32, 1999, p. 2322; “Metallocene-Based Branch-Block Thermoplastic Elastomers”, E. J. Markel, W. Weng, A. J. Peacock, and A. H. Dekmezian, Macromol., 33, 2000, pp. 8541-8548; and A. E. Cherian, E. B. Lobkovski, and G. W. Coates, Macromol., 38, 2005, pp. 6259-6268.
Others have suggested in-situ variations where the macromonomer is produced in the same reactor where the polymerization occurs, such that the macromonomer is consumed as it is produced. Examples include U.S. Pat. Nos. 7,294,681 and 7,223,822, and U.S. Patent Application Publication No. US 2004/0127614, as well as tandem polymerization catalysts such as discussed by Bazan and coworkers (Chemical Rev, 2005, 105, pp. 1001-1020 and references therein). In many cases, long chain branched polyolefins can be produced in-situ under conditions that favor macromonomer production and its reincorporation in subsequently growing chains (See Chemical Rev, 2005, 105, pp. 1001-1020 and references therein).
Polyethylene formed through macromonomer reinsertion is often referred as to long chain branched (LCB) polyethylene since most of the macromonomers have a chain length longer than the critical entanglement chain length of a linear polymer chain. As the branch length increases, they are able to form lamellar crystals of their own. LCB has a tremendous effect on the melt rheological behavior. Even very small quantities of long-chain branching (LCB) alter the polymer processing properties significantly.
In other areas, low molecular weight macromonomers of larger monomers (often referred to as polyalphaolefins), such as octene, decene and dodecene, have been made for uses in lubricants and additives. For examples please see PCT Publication No. WO 2007/011459 A1 and U.S. Pat. No. 6,706,828. Others have made various polyalphaolefins, such as polydecene, using various metallocene catalysts not typically known to produce polymers or macromonomers with any specific tacticity. Examples include PCT Publication No. WO 96/23751, European Publication No. EP 0 613 873, U.S. Patent Application Publication No. US 2003/0055184, and U.S. Pat. Nos. 5,688,887; 6,043,401; 6,548,724; 5,087,788; 6,414,090; 6,414,091; 4,704,491; 6,133,209; and 6,713,438. Many of these polyalphaolefin molecules have terminal unsaturation that is typically hydrogenated or functionalized prior to use as a lubricant or fuel additive.
There is a gap in the spectrum with respect of comonomer chain length for LLDPE applications. Low molecular weight alpha olefins such as 1-butene, 1-hexene and 1-octene are typically used in LLDPE with SCB. The chain length of macromonomers used for LCB is typically longer than the entanglement chain length. Macromonomers with intermediate molecular weight have not been explored due to lack of availability in commercial scale. Commercially produced alpha-olefins are typically made by ethylene oligomerization. U.S. Pat. No. 8,076,524 discloses an oligomerization process for trimerization and/or tetramerization of C2 to C12 olefins, preferably alpha-olefins, preferably ethylene, using the ligand-metal-precursor combinations, metal-ligand complexes, and/or catalyst systems described herein in the unique processes for generating comonomer described herein. U.S. Patent Application Publication No. US 2009/0318644 discloses a process to make high vinyl terminated propylene based macromonomers. U.S. Patent Application Publication No. US 2012/0245312 discloses a catalyst system to produce vinyl terminated high olefin macromonomers. But no attempt has been made to use the macromonomers as a comonomer in polyethylene applications. LLDPE can benefit from the addition of longer-chain comonomers. Accordingly, there is a need for new products and processes that produce LLDPE with comonomers of intermediate molecular weight.
Allyl terminated low molecular weight solids and liquids of ethylene or propylene have also been produced, typically for use as branches in polymerization reactions. See, for example, Rulhoff, Sascha, and Kaminsky, (“Synthesis and Characterization of Defined Branched Poly(propylene)s with Different Microstructures by Copolymerization of Propylene and Linear Ethylene Oligomers (Cn=26-28) with Metallocenes/MAO Catalysts”, Macromolecules, 16, 2006, pp. 1450-1460), and Kaneyoshi, Hiromu et al. (“Synthesis of Block and Graft Copolymers with Linear Polyethylene Segments by Combination of Degenerative Transfer Coordination Polymerization and Atom Transfer Radical Polymerization,” Macromolecules, 38, 2005, pp. 5425-5435).
Further, U.S. Pat. No. 4,814,540 discloses bis(pentamethyl cyclopentadienyl) hafnium dichloride, bis(pentamethyl cyclopentadienyl) zirconium dichloride and bis(tetramethyl n-butyl cyclopentadienyl) hafnium dichloride with methylalumoxane in toluene or hexane with or without hydrogen to make allylic vinyl terminated propylene homo-oligomers having a low degree of polymerization of 2-10. These oligomers do not have high Mn's and do not have at least 93% allylic vinyl unsaturation. Likewise, these oligomers lack comonomer and are produced at low productivities with a large excess of alumoxane (molar ratio≧600 Al/M; M=Zr, Hf). Additionally, no less than 60 wt % solvent (solvent+propylene basis) is present in all of the examples.
Teuben et al. (J. Mol. Catal., 62, 1990, pp. 277-287) discloses the use of [Cp*2MMe(THT)]+[BPh4] (M=Zr and Hf; Cp*=pentamethylcyclopentadienyl; Me=methyl, Ph=phenyl; THT=tetrahydrothiophene), to make propylene macromonomers. For M=Zr, a broad product distribution with macromonomers up to C24 (number average molecular weight (Mn) of 336) was obtained at room temperature. Whereas, for M=Hf, only the dimer 4-methyl-1-pentene and the trimer 4,6-dimethyl-1-heptene were formed. The dominant termination mechanism appeared to be beta-methyl transfer from the growing chain back to the metal center, as was demonstrated by deuterium labeling studies.
X. Yang et al. (Angew. Chem. Intl Ed. Engl., 31, 1992, pp. 1375-1377) disclose amorphous, low molecular weight polypropylene made at low temperatures where the reactions showed low activity and product having 90% allylic vinyls, relative to all unsaturations, by 1H NMR. Thereafter, Resconi et al. (J. Am. Chem. Soc., 114, 1992, pp. 1025-1032), discloses the use of bis(pentamethylcyclopentadienyl)zirconium and bis(pentamethylcyclopentadienyl)hafnium to polymerize propylene and obtained beta-methyl termination resulting in macromonomers and low molecular weight polymers with “mainly allyl- and iso-butyl-terminated” chains. As is the case in U.S. Pat. No. 4,814,540, the macromonomers produced do not have at least 93% allyl chain ends, an Mn of about 500 to about 20,000 g/mol (as measured by 1H NMR), and the catalyst has low productivity (1-12,620 g/mmol metallocene/hr; >3000 wppm Al in products).
Similarly, Small and Brookhart (Macromolecules, 32, 1999, pp. 2120-2130) disclose the use of a pyridylbisamido iron catalyst in a low temperature polymerization to produce low molecular weight amorphous propylene materials apparently having predominant or exclusive 2,1 chain growth, chain termination via beta-hydride elimination, and high amounts of vinyl end groups.
Weng et al. (Macromol Rapid Comm., 2000, 21, pp. 1103-1107) discloses materials with up to about 81 percent vinyl termination made using dimethylsilyl bis(2-methyl, 4-phenyl-indenyl) zirconium dichloride and methylalumoxane in toluene at about 120° C. The materials have a Mn of about 12,300 (measured with 1H NMR) and a melting point of about 143° C.
Macromolecules, 33, 2000, pp. 8541-8548 discloses preparation of branch-block ethylene-butene polymer by reincorporation of vinyl terminated polyethylene, said branch-block polymer made by a combination of CP2ZrCL2 and (C5Me4SiMe2NC12H23)TiCl2 activated with methylalumoxane.
Moscardi et al. (Organometallics, 20, 2001, pp. 1918-1931) disclose the use of rac-dimethylsilylmethylenebis(3-t-butyl indenyl)zirconium dichloride with methylalumoxane in batch polymerizations of propylene to produce materials where “ . . . allyl end group always prevails over any other end groups, at any [propene].” In these reactions, morphology control was limited and approximately 60% of the chain ends are allylic.
Coates et al. (Macromolecules, 38, 2005, pp. 6259-6268) disclose preparation of low molecular weight syndiotactic polypropylene ([rrrr]=0.46-0.93) with about 100% allyl end groups using bis(phenoxyimine)titanium dichloride ((PHI)2TiCl2) activated with modified methyl alumoxane (MMAO; Al/Ti molar ratio=200) in batch polymerizations run between −20 and +20° C. for four hours. For these polymerizations, propylene was dissolved in toluene to create a 1.65 M toluene solution. Catalyst productivity was very low (0.95 to 1.14 g/mmol Ti/hr).
Japanese Publication No. JP 2005-336092 A2 discloses the manufacture of vinyl-terminated propylene polymers using materials such as H2SO4 treated montmorillonite, triethylaluminum, triisopropyl aluminum, where the liquid propylene is fed into a catalyst slurry in toluene. This process produces substantially isotactic macromonomers that do not have a significant amount of amorphous material.
Rose et al. (Macromolecules, 41, 2008, pp. 559-567) discloses poly(ethylene-co-propylene) macromonomers not having significant amounts of iso-butyl chain ends. Those were made with bis(phenoxyimine) titanium dichloride ((PHI)2TiCl2) activated with modified methylalumoxane (MMAO; Al/Ti molar ratio range 150 to 292) in semi-batch polymerizations (30 psi propylene added to toluene at 0° C. for 30 min, followed by ethylene gas flow at 32 psi of over-pressure at about 0° C. for polymerization times of 2.3 to 4 hours to produce E-P copolymer having an Mn of about 4,800 to 23,300. In four reported copolymerizations, allylic chain ends decreased with increasing ethylene incorporation roughly according to the equation:% allylic chain ends(of total unsaturations)=−0.95(mol % ethylene incorporated)+100.For example, 65% allyl (compared to total unsaturation) was reported for E-P copolymer containing 29 mol % ethylene. This is the highest allyl population achieved. For 64 mol % incorporated ethylene, only 42% of the unsaturations are allylic. Productivity of these polymerizations ranged from 0.78×102 g/mmol Ti/hr to 4.62×102 g/mmol Ti/hr. Prior to this work, Zhu et al. reported only low (˜38%) vinyl terminated ethylene-propylene copolymer made with the constrained geometry metallocene catalyst [C5Me4(SiMe2N-tert-butyl)TiMe2 activated with B(C6F5)3 and MMAO (Macromolecules, 35, 2002, pp. 10062-10070 and Macromolecules Rap. Commun., 24, 2003, pp. 311-315).
Janiak and Blank summarize a variety of work related to oligomerization of olefins (Macromol. Symp., 236, 2006, pp. 14-22).
Schobel, Lanzinger and Reiger in “Polymerization Behavior of C1-Symmetric Metallocenes (M=Zr, Hf): From Ultrahigh Molecular Weight Elastic Polypropylene to Useful Macromonomers” (OrganoMetallics, Jan. 15, 2013) discloses propylene macromonomers containing vinyl groups used to make polyethylene-g-polypropylene copolymers.
Additional references of interest include U.S. Pat. Nos. 6,111,027; 7,183,359; 6,100,224; and 5,616,153.
Accordingly, there remains a need for ethylene copolymers with improved properties.